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std::span(3) C++ Standard Libary std::span(3) NAME std::span - std::span Synopsis Defined in header <span> template< class T, (since C++20) std::size_t Extent = std::dynamic_extent > class span; The class template span describes an object that can refer to a con- tiguous sequence of objects with the first element of the sequence at position zero. A span can either have a static extent, in which case the number of elements in the sequence is known at compile-time and encoded in the type, or a dynamic extent. If a span has dynamic extent a typical implementation holds two mem- bers: a pointer to T and a size. A span with static extent may have only one member: a pointer to T. Every specialization of std::span is a TriviallyCopyable type. (since C++23) Template parameters T - element type; must be a complete object type that is not an abstract class type Extent - the number of elements in the sequence, or std::dynamic_ex- tent if dynamic Member types Member type Definition element_type T value_type std::remove_cv_t<T> size_type std::size_t difference_type std::ptrdiff_t pointer T* const_pointer const T* reference T& const_reference const T& implementation-defined LegacyRandomAccessIterator, iterator ConstexprIterator, and contiguous_iterator whose value_type is value_type reverse_iterator std::reverse_iterator<iterator> Note: iterator is a mutable iterator if T is not const-qualified. All requirements on the iterator types of a Container apply to the iterator type of span as well. Member constant static constexpr std::size_t extent = Extent; Member functions constructor constructs a span (public member function) operator= assigns a span (public member function) Iterators begin returns an iterator to the beginning (C++20) (public member function) end returns an iterator to the end (C++20) (public member function) rbegin returns a reverse iterator to the beginning (C++20) (public member function) rend returns a reverse iterator to the end (C++20) (public member function) Element access front access the first element (C++20) (public member function) back access the last element (C++20) (public member function) operator[] accesses an element of the sequence (public member function) data returns a pointer to the beginning of the sequence of elements (public member function) Observers size returns the number of elements in the sequence (public member function) size_bytes returns the size of the sequence in bytes (public member function) empty checks if the sequence is empty (public member function) Subviews first obtains a subspan consisting of the first N elements of the sequence (public member function) last obtains a subspan consisting of the last N elements of the sequence (public member function) subspan obtains a subspan (public member function) Non-member functions as_bytes converts a span into a view of its underlying bytes as_writable_bytes (function template) (C++20) Non-member constant dynamic_extent a constant of type size_t signifying that the span has dynamic extent (C++20) (constant) Helper templates template<class T, std::size_t Extent> inline constexpr bool ranges::enable_borrowed_range<std::span<T, Ex- tent>> = true; This specialization of ranges::enable_borrowed_range makes span sat- isfy borrowed_range. template<class T, std::size_t Extent> inline constexpr bool ranges::enable_view<std::span<T, Extent>> = true; This specialization of ranges::enable_view makes span satisfy view. Deduction guides Notes Specializations of std::span are already trivially copyable types in all existing implementations, even before the formal requirement introduced in C++23. Feature-test macro: __cpp_lib_span Example The example uses std::span to implement some algorithms on contigu- ous ranges. // Run this code #include <algorithm> #include <cstddef> #include <iostream> #include <span> template<class T, std::size_t N> [[nodiscard]] constexpr auto slide(std::span<T,N> s, std::size_t offset, std::size_t width) { return s.subspan(offset, offset + width <= s.size() ? width : 0U); } template<class T, std::size_t N, std::size_t M> [[nodiscard]] constexpr bool starts_with(std::span<T,N> data, std::span<T,M> prefix) { return data.size() >= prefix.size() && std::equal(prefix.begin(), prefix.end(), data.begin()); } template<class T, std::size_t N, std::size_t M> [[nodiscard]] constexpr bool ends_with(std::span<T,N> data, std::span<T,M> suffix) { return data.size() >= suffix.size() && std::equal(data.end() - suffix.size(), data.end(), suffix.end() - suffix.size()); } template<class T, std::size_t N, std::size_t M> [[nodiscard]] constexpr bool contains(std::span<T,N> span, std::span<T,M> sub) { return std::search(span.begin(), span.end(), sub.begin(), sub.end()) != span.end(); // return std::ranges::search(span, sub).begin() != span.end(); } void print(const auto& seq) { for (const auto& elem : seq) std::cout << elem << ' '; std::cout << '\n'; } int main() { constexpr int a[] { 0, 1, 2, 3, 4, 5, 6, 7, 8 }; constexpr int b[] { 8, 7, 6 }; for (std::size_t offset{}; ; ++offset) { static constexpr std::size_t width{6}; auto s = slide(std::span{a}, offset, width); if (s.empty()) break; print(s); } static_assert( starts_with( std::span{a}, std::span{a, 4} ) and starts_with( std::span{a + 1, 4}, std::span{a + 1, 3} ) and ! starts_with( std::span{a}, std::span{b} ) and ! starts_with( std::span{a, 8}, std::span{a + 1, 3} ) and ends_with( std::span{a}, std::span{a + 6, 3} ) and ! ends_with( std::span{a}, std::span{a + 6, 2} ) and contains( std::span{a}, std::span{a + 1, 4} ) and ! contains( std::span{a, 8}, std::span{a, 9} ) ); } Output: 0 1 2 3 4 5 1 2 3 4 5 6 2 3 4 5 6 7 3 4 5 6 7 8 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct be- havior P2325R3 C++20 span of non-zero static they are as default_ini- tializable is not extents were not view required See also initializer_list creates a temporary array in list-initialization and then (C++11) references it (class template) basic_string_view read-only string view (C++17) (class template) ranges::subrange combines an iterator-sentinel pair into a view (C++20) (class template) http://cppreference.com 2022.07.31 std::span(3)
NAME | Synopsis | Template parameters | Member types | Member functions | Iterators | Element access | Observers | Non-member functions | Notes | Example | Output: | See also
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